CIRCADIAN CHANGES IN THE SENSITIVITY OF THE CORTICOTROPIN-RELEASING HORMONE-STIMULATED HPA SYSTEM AFTER ARGININE-VASOPRESSIN AND ATRIAL NATRIURETIC HORMONE IN HUMAN MALE CONTROLS

In order to investigate whether modulation of the effects of corticotr opin-releasing hormone (CRH) by arginine vasopressin (AVP) and atrial natriuretic hormone (ANH) is sensitive to circadian fluctuations, 12 h ealthy male volunteers were administered 100 mu g hCRH during 30 min i nfusion of placebo, 3 IU AVP, or 150 mu g ANH, the experiment being ca rried out in the morning (0800h) or in the evening (1900h). Compared w ith placebo, AVP significantly amplified the CRH-induced adrenocortico tropin hormone (ACTH) surge both in the morning and in the evening, wh ereas it enhanced cortisol secretion only in the evening. In contrast, ANH significantly reduced the ACTH surge, but only in the evening, an d it reduced the cortisol surge in the morning compared to the evening . During ANH infusion plasma AVP levels were suppressed both in the mo rning and in the evening, pointing to a hypothalamic site of action in addition to the pituitary and adrenocortical effects. The ACTH secret ion profiles were indistinguishable between morning and evening in all three experimental conditions, whereas cortisol profiles differed sig nificantly depending on the time of day in the AVP and ANH condition, but not for placebo.